Other times, it's because the health department ordered an inquiry after receiving a food-poisoning complaint. And a new study from the Centers for Disease Control and Prevention, focusing on Chicago, suggests that when customers complain about a restaurant's health standards, they're often right.

The study tracked 133 restaurant inspections that took place after people who had eaten in them reported suspected food poisoning to the city through an online form. Twenty-one of the restaurants failed the reviews, and 33 more passed only after fixing major violations that posed health hazards.

Lee said people who suspect they have food poisoning after eating at a San Francisco restaurant or other food vendor should call 3-1-1. Health department staff members then interview the caller and inspect the restaurant in question. Restaurants are legally required to post the results of their latest health inspections, and diners can find out how their favorite restaurant performed in the city's online database: http://bit.ly/1p104rW

Here are the numbers:

5,500

The number of restaurants, bars, bakeries, pushcarts, stadium food outlets and other food vendors Lee said the San Francisco Department of Public Health inspects twice each year. There are an additional 2,100 places that sell food but don't prepare it, and those are inspected once a year.

218

The number of food-borne illness complaints the San Francisco health department received in 2013.

202

The number of restaurants or similar businesses the San Francisco health department shut down in 2013 until corrections could be made because they posed "an imminent health hazard." Lee said the department doesn't track how many of these closures can be linked back to food-borne illness complaints from customers.

105 million

The number of Americans the CDC estimates experience food poisoning each year, though the actual number is tough to know because many people don't report the illness.

16 percent

The percentage of the restaurants inspected in the CDC study that failed inspection after diners reported them to Chicago's health department. An additional 25 percent passed only after correcting violations.

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